The most promising approach to restenosis prevention is the placement of a stent in the blood vessel which has undergone balloon angioplasty at the position in the vessel where the balloon was inflated immediately following removal of the balloon. The term stent is now in common use to denote a short tube, open at both ends for insertion in a blood vessel following balloon angioplasty to prevent restenosis, terms other than stent such as graft prosthesis, arterial endoprothesis, intraluminal graft and intravascular mechanical support may be and are frequently used instead of "stent" to convey the same meaning.
Dotter U.S. Pat. No. 4,503,569 for Transluminally Placed Expandable Graft Prosthesis describes a stent in detail. A Dotter et al. paper titled Transluminally Expendable Coil Stent Grafting was published in Radiology 147:25960 a month after his patent application was filed. Paper and patent are directed to the same subject matter. It is of passing interest that a Cragg et al. paper titled Nonsurgical Placement of Arterial Endoprotheses: A New Technique Using Nitinol Wire was published in Radiology 147:261-263 and is essentially identical in content with the Dotter et al. paper.
Both papers and the patent describe the making of a stent from nitinol, a shape memory alloy of titanium and nickel. The stent is a short helical coil of nitinol wire. The diameter of the helical coil is equal to or slightly greater than that of the blood vessel in which it is intended to use the stent. After the helical coil is made it is heated to fix the shape of the coil in the memory of the nitinol. The wire of the helical coil is then wound to form a helical coil having an appreciably smaller diameter than that of the first helical coil. The smaller diameter coil is then placed in the blood vessel at the place where the balloon was inflated. After placement in the blood vessel the coil is heated by passing warm (115.degree.-125.degree. F.) saline solution thru a catheter to heat the stent. Upon being heated the metal of the stent returns to its first larger diameter shape and presses firmly against the blood vessel walls where it is left to hold the blood vessel open preventing restenosis.
The term "nitinol" as used in the prior art, e.g. Dotter and Cragg, is not used to identify an alloy of fully specified composition, but rather to a "nitinol family" of alloys, all the members of which consist principally of nickel and titanium in varying proportions and many of which contain minor amounts of another element or elements to vary the properties of the alloy.
Shape memory alloys, also called "marmem" alloys, are materials which if used to form an article of particular shape which article is heated to elevated temperature, e.g. 500.degree. C., and held at the elevated temperature for a short time period, e.g. 30 minutes, and then cooled at least to ambient temperature, retains a "memory" of the particular shape. If the article is them deformed or reshaped the memory of the particular shape remains with it and if the reshaped article is heated to a moderately elevated temperature, e.g. 90.degree.-140.degree. F., the reshaped article returns to the original particular shape. The moderately elevated temperature range at which the article returns to its original shape is called the Transition Temperature Range of the particular alloy. Both Dotter et al. and Cragg et al. used this property of nitinol to advantage.
Both Dotter et al. and Cragg et al., in their papers expressed the opinion that nitinol in addition to its shape memory which facilitated fixing a nitinol stent at a position in the blood vessel was resistant to formations of thrombi on its surface. Cragg et al. summarized their view saying, "By using Nitinol wire, the two main problems associated with this technique, namely, thrombosis of the endoprothesis and difficulty in introducing a graft of suitable size through conventional angiographic catheters, appear to be solved."
Since the publication of the Cragg et al. and Dotter et al. papers in April 1983 further experience with nitinol prostheses has shown that thrombus formation does occur on the surface of nitinol stents planted in the arteries. ( Wright et al. Radiology 1985; 156:69-72).